1. Field of the Invention
The present invention relates to a method for evaluating the cleanliness of metallic materials. More specifically, the present invention relates to a method for evaluating the cleanliness of a metallic material to be tested by scanning predetermined inspection fields of the metallic material under test by ultrasonic flaw detection method to obtain data on non-metallic inclusions (for example, oxide, nitride, sulfide, etc.) included therein, and calculating estimated maximum non-metallic inclusion diameter in the metallic material to be tested from these data with predetermined equations.
The present invention also relates to a metallic material affixed with the evaluation of cleanliness described above.
2. Description of the Related Art
Recently, with the advancement in the metallurgical technology, cleanliness of metallic materials such as steel has been greatly improved, so that non-metallic inclusions of medium to large sizes beyond 20 .mu.m are rarely included in metallic materials and sizes of the non-metallic inclusions present have also been decreasing. In this trend, it is becoming very difficult to detect large inclusions that occur accidentally or with an extremely low probability.
There is no technique capable of evaluating and ensuring cleanliness of metallic materials practically to cope with such a situation as described above.
A method for testing cleanliness of metallic materials that is currently a standard practice employs optical microscope observation. However, the area that can be covered with this method is as small as 1000 mm.sup.2, thus making the method utterly impractical for evaluating the cleanliness of metallic materials of high cleanliness as mentioned above (JIS G0555, ASTM E45, ASTMA 295, DIN 50602, ISO 4967, etc.).
On the other hand, such methods have been proposed that inclusions are extracted from a metallic material by acid dissolution with the particle size of the inclusion being evaluated under a microscope, or a metallic material is dissolved by Electron Beam Melting dissolution with the inclusions that float being observed under a microscope (Japanese Patent Application Laid-open No. Hei 9-125199, Japanese Patent Application Laid-open No. Hei 9-125200). However, in certain cases, as the inclusions dissolve into the acid or the inclusion itself fuses or coagulates, these methods also cannot be applied to the evaluation of cleanliness of metallic materials of high cleanliness.
The methods described above have also such a drawback as the inability to process quickly due to a long time required for the acid dissolution or other cause, and therefore are difficult to adapt to mass production processes.
In an industry where a metallic material of high cleanliness is processed as a raw material, strict machining processes are designed on the basis of assumption that the metallic material has a high cleanliness. When there are variations in the cleanliness of the metallic material, however, such problems as frequent occurrences of defective products result, thus leading to significant decrease in productivity.
In bearing steel, spring steel, steel to make power transmission shaft, gear steel and the like that bear high loads, for example, immature breakage may result from the presence of inclusions of sizes larger than a certain level. In such industries, therefore, there are demands for the supply of metallic materials that bear certification of high cleanliness thereof or that are guaranteed to have a certain level of cleanliness.